As shown in FIG. 8, the metal oxide film resistor generally comprises a rod-like insulating substrate 1 of mullite, alumina or the like, a metal oxide film 10 of tin oxide or antimony doped tin oxide (ATO) which is formed on the surface of said substrate, metallic cap terminals 5 and 6 which are pressed in both ends of said substrate, leads 7 and 8 welded to said terminals, and a protective film 9 formed on the surface of the resistor.
By the way, considering materials which can be used as a metal oxide film, the single phase of tin oxide has a too large resistivity and an extremely large negative temperature coefficient of resistance, and therefore, the using conditions are strictly limited and said single phase of tin oxide has no practical use. For such reasons, generally, ATO having a small resistivity and a TCR of positive or nearly 0 is put to practical use as a material of the metal oxide film. In these materials, the carrier density is high. During the rise of the temperature, the scattering effect of the carrier due to the lattice vibrations is larger than the increase of the carrier density due to the excitation energy of heat, and therefore, the TCR is positive and the metallic electric conduction can be obtained. Thus, generally, the material having a small resistivity has a high carrier density and a TCR which is positive or nearly 0. On the other hand, the material having a large resistivity has a low carrier density and a TCR which is negative and large.
The method for producing a metal oxide film resistor as described above generally includes a chemical process for forming a film such as a spraying and a chemical vapor deposition. According to these methods, the vapor of aqueous solution or organic solution containing stannic chloride and antimony trichloride is atomized to the rod-like substrate 1 of mullite-alumina in the furnace in which the temperature is 600.degree. to 800.degree. C., to form ATO film (metal oxide film 10) on the surface of the substrate. Then, the metallic cap terminal 5 and 6 are pressed in both ends of the substrate 1. While the substrate is rotated, a part of ATO film is trimmed with a diamond cutter or a laser to obtain the desired resistance. The leads 7 and 8 are welded to the cap terminals 5 and 6. Thereafter, the protective film 9 of resin is formed to obtain a metal oxide film resistor. The final resistance of the resulting metal oxide film resistor as achieved in such a way is generally 10 .OMEGA. to 100 k .OMEGA. depending on the thickness of ATO film and the turn number of trimming in the case that the size of the substrate is constant.
According to the conventional method for regulating resistance, in order to obtain a metal oxide film resistor having a final resistance of 100 k .OMEGA. and more, the thickness of ATO film may be reduced or the interval of trimming may be narrow.
However, in the case of the conventional construction, the resistivity of ATO film is about 1.times.10.sup.-3 .about.1.times.10.sup.-2 .OMEGA..multidot.cm, and therefore, the thickness of the film must be reduced considerably to raise the resistance value. At this time, because of the distortion of the film itself and the increase of the ratio of the depletion layer in the surface of the film to the whole film, there was a problem that TCR was liable to be negative and large.
Further, because of the low initial resistance of ATO film, in the case of requiring the final resistance of 100 k .OMEGA. and more, the turn number of the trimming with a laser must be increased, with the result that the trimming requires extremely much time and the interval of trimming is too narrow. And therefore, there was another problem that the trimming of the film was physically impossible.
As described above, if the thickness of the film is too thin or the interval of the trimming is too narrow, the cross-sectional area of the electric conduction path is reduced and the area in contact with the outside is increased. Because of electrical stress, humidity or the like, the resistance of the film itself changes under the influence of moisture and the alkali ions in the insulating substrate. And therefore, it was difficult to obtain a reliable metal oxide film resistor.
Then, it is an object of the present invention to provide a reliable metal oxide film resistor which is not influenced by moisture or the alkali ions in the insulating substrate and in which the resistance of the film itself does not change.